This invention relates to closures for containers, and more particularly to "safety" closures having a locking feature so that when sealingly engaged on a container, the cap is locked thereon and can be removed only by special manipulation.
Safety closures are designed to lock onto a container in such a fashion as to make their removal difficult, particularly for children. Safety closures are typically used in connection with containers for drugs, medicines, corrosive materials and the like which can cause serious injury. For example, it is required that certain pharmaceuticals be dispensed in containers provided with safety closures in order to avoid accidental opening.
Several designs of safety closures are available including single piece and multi-piece designs. The single piece closure comprises a single closure having a thread design typically consisting of a plurality of separate projections or lugs which are adapted to engage with or between corresponding indentations or lugs on a container so that the closure is locked into position thereon. The closures are removed by manipulating the cap to disengage the lugs and free the closure. An example of the multi-piece design are closures designed with conventional threading and which are provided with an outer member, such as for example, a freely rotatable outer ring member, so that the closure can be removed only by exerting a substantial force to frictionally engage the ring and the closure in order to permit rotation of the closure portion. The single piece closures having a locking thread design are preferred as they are easier to manufacture and are difficult to remove without special manipulation.
Examples of the various designs for safety closures are to be found in U.S. Pat. No. 3,072,276 (Nickols), No. 3,360,147 (Shaeffer) and No. 2,776,066 (Thornton). A significant problem encountered with prior art safety closures is that when employing a multiplicity of projections, or lugs, as the locking means it has been found that they can often be easily sheared, thus neutralizing or substantially reducing the effectiveness of the safety locking feature. Also, in some prior art designs, it is necessary that lug or projection and the closure be formed from a resilient material so as to provide the necessary flexibility to permit the closure to be urged into and out of its locked position. Such designs are unsuitable for use with metal, glass, rigid plastic, or the like, which materials are often preferred for use as closure compositions.
Cooke, in U.S. Pat. No. 3,888,376 and U.S. Pat. No. 3,942,899 discloses an improved closure design employing a substantially continuous thread in which the locking means are integrally formed as part of the thread. In this manner the shear strength of the locking means is increased and the probability of removing the closure without the proper manipulative steps by shearing the locking means is substantially reduced. Typically, however, only one or possibly two locking means are integrally formed on the threads and it has been found that while being urged into the locking position, uneven pressure is exerted between the closure and the container end which results in uneven compression on the resilient member and loss of reliability of its sealing function.